According to a study
published in the January 2001 edition of Environmental Health Perspectives
(Vol. 109, No. 1), 84% of wild fall chinook salmon (Oncorhynchus tshawytcha)
phenotypic females that returned to spawn in the Columbia River had
a genetic marker found only on the Y (male) chromosome.

As in humans, sex
in salmon is determined chromosomally: females are XX and males are
XY. The researchers, based out of the University of Idaho and Washington
State University, tested for the presence of a genetic marker present
only in XY individuals. Under normal circumstances, an individual with
this genetic marker would grow up male, suggesting that 84% of the wild
Chinook salmon were sex-reversed. None of the hatchery-raised salmon
sampled demonstrated this abnormality.

The researchers
believe the sex reversal is most likely due to the unnaturally high
water temperatures induced by dams in the Columbia River basin and/or
endocrine disrupting chemicals, including pesticides, polluting the
Columbia River. The feminization of male fish by endrocrine disrupting
chemicals was first discovered in England in the early 1990's. Subsequent
studies have shown this phenomenon in the United States. Scientists
believe that the feminization of male salmon and the resulting inability
to reproduce is most likely contributing to the dwindling endangered
salmon population in the Pacific Northwest.

To read the abstract
visit http://ehpnet1.niehs.nih.gov/docs/2001/109p67-69nagler/abstract.html.